Serine-borate complex as a transition-state inhibitor of gamma-glutamyl transpeptidase

Effects of landfill leachate effluent and bisphenol A on glutathione and glutathione-related enzymes in the gills and digestive glands of the freshwater snail Bellamya purificata

Environmental contaminants with estrogenic activity have recently attracted attention due to their potential detrimental effects on the reproduction of human and wildlife. The aim of this study was to evaluate the use of endogenous glutathione and glutathione-related enzymes as biomarkers of exposure to landfill leachate effluent and bisphenol A (BPA) in the freshwater snail, Bellamya purificata. Following exposure to 1%, 5% and 10% landfill leachate effluent and 1, 10, 50 and 100mugl(-1) BPA for 0, 2, 7 and 15d, activities of glutathione S-transferase (GST), selenium-dependent glutathione peroxidase (SeGPx) and glutathione reductase (GR) and levels of total glutathione were measured in the gills and digestive glands of the snails. GST and total glutathione were the most sensitive parameters in both exposure scenarios. GST activities increased by about 80%, while total glutathione decreased to 70% and 80% in the gills and digestive glands, respectively. In contrast, SeGPx and GR activities remained at the same levels in all the treatment groups compared with those of controls. The results indicated that among glutathione and glutathione-related enzymes, GST activity and total glutathione level, which showed dose-dependent dynamics, could be used as biomarkers of aquatic ecosystems contaminated with landfill leachate.

Localization of gamma-glutamyl transferase activity and protein in Zea mays organs and tissues

Gamma-glutamyl transferase/transpeptidase (GGT, (5-l-glutamyl)-peptide:amino-acid 5-glutamyltransferase; EC 2.3.2.2.) is an ectoenzyme promoting the cleavage of the gamma-glutamyl moiety of glutathione (GSH) and gamma-glutamyl related compounds. In this work, we describe the localization of GGT by enzymehistochemical and immunohistochemical analysis in maize plants. Our results show that the tissue spatial distribution of GGT activity closely correlates with the localization of the GGT protein. We also demonstrate that GGT activity and protein are unevenly distributed in tissues, being higher in the epidermis and stomata, parenchyma of conductive elements and root meristem. These results can contribute to our understanding of GGT function and regulation as well as its role in glutathione metabolism. To date, these are largely unknown in plants.

Ternary borate-nucleoside complex stabilization by ribonuclease A demonstrates phosphate mimicry

Phosphate esters play a central role in cellular energetics, biochemical activation, signal transduction and conformational switching. The structural homology of the borate anion with phosphate, combined with its ability to spontaneously esterify hydroxyl groups, suggested that phosphate ester recognition sites on proteins might exhibit significant affinity for nonenzymatically formed borate esters. (11)B NMR studies and activity measurements on ribonuclease A (RNase A) in the presence of borate and several cytidine analogs demonstrate the formation of a stable ternary RNase A.3'-deoxycytidine-2'-borate ternary complex that mimics the complex formed between RNase A and a 2'-cytidine monophosphate (2'-CMP) inhibitor. Alternatively, no slowly exchanging borate resonance is observed for a ternary RNase A, borate, 2'-deoxycytidine mixture, demonstrating the critical importance of the 2'-hydroxyl group for complex formation. Titration of the ternary complex with 2'-CMP shows that it can displace the bound borate ester with a binding constant that is close to the reported inhibition constant of RNase A by 2'-CMP. RNase A binding of a cyclic cytidine-2',3'-borate ester, which is a structural homolog of the cytidine-2',3'-cyclic phosphate substrate, could also be demonstrated. The apparent dissociation constant for the cytidine-2',3'-borate.RNase A complex is 0.8 mM, which compares with a Michaelis constant of 11 mM for cytidine-2',3'-cyclic phosphate at pH 7, indicating considerably stronger binding. However, the value is 1,000-fold larger than the reported dissociation constant of the RNase A complex with uridine-vanadate. These results are consistent with recent reports suggesting that in situ formation of borate esters that mimic the corresponding phosphate esters supports enzyme catalysis.

Low dietary boron reduces parasite (nematoda) survival and alters cytokine profiles but the infection modifies liver minerals in mice

Although boron (B) is an essential trace mineral, any interactions that it may have with gastrointestinal (GI) nematode infections are unknown. This study explored whether low dietary B would: 1) alter survival or reproduction of Heligmosomoides bakeri (Nematoda); 2) modify the resulting cytokine response to this parasitic infection; or 3) influence liver mineral concentrations in the infected host. Balb/c mice were fed either a low-B (0.2 microg B/g), marginal (2.0 microg B/g), or control (12.0 microg B/g) diet. Diets commenced 3 wk before a primary infection and were fed for 4 wk (primary infection protocol) and 8-9 wk (challenge infection protocol). Mice were killed 6 d post-primary infection (d6ppi), or dewormed then reinfected (challenge infection protocol) and killed 14 or 21 d post-challenge infection (d14pci or d21pci, respectively). Low and marginal dietary B intakes impaired survival of the parasite, reduced intestinal inflammation, and modulated a broad range of cytokines and chemokines despite similar liver B concentrations in diet groups. Compared with control mice, cytokine production was lower following low and marginal B intakes at d6ppi but was elevated at d21pci. Serum alkaline phosphatase was higher at d6ppi than at d14pci and d21pci. Compared with d14pci, liver zinc, iron, and B concentrations were reduced at d21pci when worm numbers were also lower, whereas concentrations of sodium, potassium, molybdenum, chromium, and sulfur were higher. This study shows that parasite survival and cytokine and inflammatory responses are modified by dietary B intake but indicates that a GI nematode infection alters liver mineral concentrations.

Identification and characterization of a gamma-glutamyl transpeptidase from a thermo-alcalophile strain of Bacillus pumilus

A gamma-glutamyltranspeptidase (GGT, E.C. 2.3.2.2) was isolated from a strain (A8) originating from Lake Bogoria (Kenya) and homologous with Bacillus pumilus. This GGT shows an optimal activity at pH 8.9 and 62 degrees C. The enzyme is thermostable up to 43 degrees C. The best reagent among the potential inhibitors was shown to be DON, which is an inhibitor highly specific for GGTs. Gly-Gly-Ala, Gly-Gly-Gly and Gly-Gly were identified as the best acceptors for the transpeptidation reactions catalyzed by the enzyme. The SDS-PAGE study revealed that the enzyme consists of two non-identical subunits (38,000 and 23,000). Only the large subunit was active when the enzyme was dissociated under denaturing conditions. The behavior of the native enzyme suggests that the active site of the large subunit is masked by the small subunit.

Gamma-(monophenyl)phosphono glutamate analogues as mechanism-based inhibitors of gamma-glutamyl transpeptidase

Gamma-glutamyl transpeptidase (GGT, EC 2.3.2.2) catalyzes the hydrolysis and transpeptidation of extracellular glutathione and plays a central role in glutathione homeostasis. We report here the synthesis and evaluation of a series of hydrolytically stable gamma-(monophenyl)phosphono glutamate analogues with varying electron-withdrawing para substituents on the leaving group phenols as mechanism-based and transition-state analogue inhibitors of Escherichia coli and human GGTs. The monophenyl phosphonates caused time-dependent and irreversible inhibition of both the E. coli and human enzymes probably by phosphonylating the catalytic Thr residue of the enzyme. The inactivation rate of E. coli GGT was highly dependent on the leaving group ability of phenols with electron-withdrawing groups substantially accelerating the rate (Brønsted betalg = -1.4), whereas the inactivation of human GGT was rather slow and almost independent on the nature of the leaving group. The inhibition potency and profiles of the phosphonate analogues were compared to those of acivicin, a classical inhibitor of GGT, suggesting that the phosphonate-based glutamate analogues served as a promising candidate for potent and selective GGT inhibitors.

Simultaneous quantification of reduced and oxidized glutathione in plasma using a two-dimensional chromatographic system with parallel porous graphitized carbon columns coupled with fluorescence and coulometric electrochemical detection

A method for the simultaneous quantification of reduced and oxidized glutathione in human plasma employing a two-dimensional chromatographic system with parallel porous graphitized carbon (PGC) columns coupled with fluorescence (FLD) and coulometric electrochemical detection (ED) has been developed. Post-sampling oxidation of reduced glutathione (GSH) was prevented by derivatizing the -SH group with monobromobimane (MBB) and the glutathione-bimane adduct (GSMB) was detected by FLD. Oxidized glutathione (GSSG) was detected by ED optimized to give lowest possible limits of detection (LOD). The method is fully validated and is currently used for determination of GSH, GSSG and its redox potential in different clinical studies.

Role of glutathione transport processes in kidney function

The kidneys are highly dependent on an adequate supply of glutathione (GSH) to maintain normal function. This is due, in part, to high rates of aerobic metabolism, particularly in the proximal tubules. Additionally, the kidneys are potentially exposed to high concentrations of oxidants and reactive electrophiles. Renal cellular concentrations of GSH are maintained by both intracellular synthesis and transport from outside the cell. Although function of specific carriers has not been definitively demonstrated, it is likely that multiple carriers are responsible for plasma membrane transport of GSH. Data suggest that the organic anion transporters OAT1 and OAT3 and the sodium-dicarboxylate 2 exchanger (SDCT2 or NaDC3) mediate uptake across the basolateral plasma membrane (BLM) and that the organic anion transporting polypeptide OATP1 and at least one of the multidrug resistance proteins mediate efflux across the brush-border plasma membrane (BBM). BLM transport may be used pharmacologically to provide renal proximal tubular cells with exogenous GSH to protect against oxidative stress whereas BBM transport functions physiologically in turnover of cellular GSH. The mitochondrial GSH pool is derived from cytoplasmic GSH by transport into the mitochondrial matrix and is mediated by the dicarboxylate and 2-oxoglutarate exchangers. Maintenance of the mitochondrial GSH pool is critical for cellular and mitochondrial redox homeostasis and is important in determining susceptibility to chemically induced apoptosis. Hence, membrane transport processes are critical to regulation of renal cellular and subcellular GSH pools and are determinants of susceptibility to cytotoxicity induced by oxidants and electrophiles.

Quaternary Ammonium Arylspiroborate Esters as Organo-Soluble, Environmentally Benign Wood Protectants

As part of a larger project aimed at the development of leach resistant boron-based wood preservatives, the anti-fungal and termiticidal activities, and the resistance to leaching from timber, of three related tetra-n-butylammonium spiroborates, tetra-n-butylammonium bis(ortho-hydroxymethylphenolato)borate 2, tetra-n-butylammonium bis[catecholato(2–)-O,O′]borate 3, and tetra-n-butylammonium bis[salicylato(2−)-O,O']borate 4, have been examined. All three borates are found to be active against test organisms, with the following orders of activity being observed: 2 > 3 > 4 > boric acid against wood decay fungi, and 2 > 3 ≈ 4 > boric acid against termites. The most active compound in both assays 2 also has the highest calculated lipophilicity. In a test for permanence in wood, the following order of leach resistance is observed: 4 >> 3 ≈ 2 > boric acid. This order appears to correlate more closely with the stability constants of the borate esters, as determined using 11B NMR spectroscopy, rather than calculated lipophilicities.

Contribution of acetaminophen-cysteine to acetaminophen nephrotoxicity II. Possible involvement of the γ-glutamyl cycle

Acetaminophen (APAP) nephrotoxicity has been observed both in humans and research animals. Our recent investigations have focused on the possible involvement of glutathione-derived APAP metabolites in APAP nephrotoxicity and have demonstrated that administration of acetaminophen-cysteine (APAP-CYS) potentiated APAP-induced renal injury with no effects on APAP-induced liver injury. Additionally, APAP-CYS treatment alone resulted in a dose-responsive renal GSH depletion. This APAP-CYS-induced renal GSH depletion could interfere with intrarenal detoxification of APAP or its toxic metabolite N-acetyl-p-benzoquinoneimine (NAPQI) and may be the mechanism responsible for the potentiation of APAP nephrotoxicity. Renal-specific GSH depletion has been demonstrated in mice and rats following administration of amino acid gamma-glutamyl acceptor substrates for gamma-glutamyl transpeptidase (gamma-GT). The present study sought to determine if APAP-CYS-induced renal glutathione depletion is the result of disruption of the gamma-glutamyl cycle through interaction with gamma-GT. The results confirmed that APAP-CYS-induced renal GSH depletion was antagonized by the gamma-glutamyl transpeptidase (gamma-GT) inhibitor acivicin. In vitro analysis demonstrated that APAP-CYS is a gamma-glutamyl acceptor for both murine and bovine renal gamma-GT. Analysis of urine from mice pretreated with acivicin and then treated with APAP, APAP-CYS, or acetaminophen-glutathione identified a gamma-glutamyl-cysteinyl-acetaminophen metabolite. These findings are consistent with the hypothesis that APAP-CYS contributes to APAP nephrotoxicity by depletion of renal GSH stores through interaction with the gamma-glutamyl cycle.

Prominent role of γ -glutamyl-transpeptidase on the growth of Helicobacter pylori

AIM: γ -glutamyl transpeptidase (GGT) has been reported as a virulence and colonizing factor of Helicobacter pylori (H pylori). This study examined the effect of GGT on the growth of H pylori.

METHODS: Standard H pylori strain NCTC 11637 and 4 clinical isolates with different levels of GGT activity as measured by an enzymatic assay were used in this study. Growth inhibition and stimulation studies were carried out by culturing H pylori in brain heart infusion broth supplemented with specific GGT inhibitor (L-serine sodium borate complex, SBC) or enhancer (glutathione together with glycyl-glycine), respectively. The growth profiles of H pylori were determined based on viable bacterial count at time interval.

RESULTS: Growth was more profuse for H pylori isolates with higher GGT activity than those present with lower GGT activity. However, in the presence of SBC, growth of H pylori was retarded in a dose dependent manner (P = 0.034). In contrast, higher growth rate was observed when GGT activity was enhanced in the presence of glutathione and glycyl-glycine.

CONCLUSION: Higher GGT activity provides an advantage to the growth of H pylori in vitro. Inhibition of GGT activity by SBC resulted in growth retardation. The study shows that GGT plays an important role on the growth of H pylori.

LTD4 induces hyperresponsiveness to histamine in bovine airway smooth muscle: role of SR-ATPase Ca2+ pump and tyrosine kinase

Airway hyperresponsiveness is a key feature of asthma, but its mechanisms remain poorly understood. Leukotriene D(4) (LTD(4)) is one of the few molecules capable of producing airway hyperresponsiveness. In this study, LTD(4), but not leukotriene C(4) (LTC(4)), produced a leftward displacement of the concentration-response curve to histamine in bovine airway smooth muscle strips. Neither LTC(4) nor LTD(4) modified the concentration-response curve to carbachol. In simultaneous measurements of intracellular Ca(2+) ([Ca(2+)](i)) and contraction, histamine or carbachol produced a transient Ca(2+) peak followed by a plateau, along with a contraction. LTD(4) increased the histamine-induced transient Ca(2+) peak and contraction but did not modify responses to carbachol. Enhanced responses to histamine induced by LTD(4) were not modified by staurosporine or chelerythrine but were abolished by genistein. Western blot showed that carbachol, but not histamine, caused intense phosphorylation of extracellular signal-regulated kinase 1/2 and that LTD(4) significantly enhanced the phosphorylation induced by histamine, but not by carbachol. L-type Ca(2+) channel participation in the hyperresponsiveness to histamine was discarded because LTD(4) did not modify the [Ca(2+)](i) changes induced by KCl. In tracheal myocytes, LTD(4) enhanced the transient Ca(2+) peak induced by histamine (but not by carbachol) and the sarcoplasmic reticulum (SR) Ca(2+) refilling. Genistein abolished this last LTD(4) effect. Partial blockade of the SR-ATPase Ca(2+) pump with cyclopiazonic acid reduced the Ca(2+) transient peak induced by histamine but not by carbachol. These results suggested that LTD(4) induces hyperresponsiveness to histamine through activation of the tyrosine kinase pathway and an increasing SR-ATPase Ca(2+) pump activity. L-type Ca(2+) channels seemed not to be involved in this phenomenon.

Use of borate to control the 5'-position-selective microbial glucosylation of pyridoxine

Nearly 100% 5'-position selectivity of transglucosylation from maltodextrin to pyridoxine (PN) by cells of Verticillium dahliae TPU 4900 was observed when the reaction was carried out with borate. The same effect of borate was observed not only during synthesis of pyridoxine 5'-alpha-D-glucoside by partially purified enzyme of this strain but also during synthesis of this compound by other microorganisms and with other enzymes (alpha-glucosidase and cyclomaltodextrin glucanotransferase). The effect was thought to be caused by the formation of a borate complex with 3- and 4'-position hydroxyl groups of PN. A decrease in the formation of pyridoxine 5'-alpha-D-glucoside was observed in the reaction with borate, but this decrease was overcome by optimizing the pH and increasing the amount of cells in the reaction mixture.

High pressure liquid chromatography and mass spectrometry characterization of the nephrotoxic biotransformation products of Cisplatin

Previous studies have shown that cisplatin requires metabolic activation to become nephrotoxic. The activation is proposed to be via the metabolism of a glutathione-platinum conjugate to a cysteinyl-glycine-platinum conjugate, which is further processed to a cysteine conjugate. Preincubating cisplatin with glutathione (GSH), cysteinyl-glycine, or N-acetylcysteine (NAC) results in a transient increase in the toxicity of cisplatin toward renal proximal tubular cells. In this study, the preincubation solutions were analyzed by high pressure liquid chromatography (HPLC), atomic absorption spectrometry, and mass spectrometry (MS) to characterize the formation and structure of the platinum conjugates. HPLC analysis of the cisplatin-GSH, cisplatin-cysteinyl-glycine, and cisplatin-NAC preincubation solutions revealed two new platinum-containing peaks in each of the solutions. MS-MS analysis of the peaks revealed a diplatinum- and a monoplatinum conjugate in each of the solutions. Analysis of the composition and toxicity of the solutions with time showed that the transient increase in toxicity correlated with the formation of the monoplatinum conjugate whereas prolonged preincubation decreased toxicity and correlated with the formation of the diplatinum conjugate. The monoplatinum-monoglutathione conjugate is a substrate for gamma-glutamyl transpeptidase, an enzyme that is essential for the nephrotoxicity of cisplatin. The monoplatinum-mono-NAC conjugate can be deacetylated to a cysteine conjugate, which is a substrate for pyroxidol phosphate (PLP)-dependent cysteine S-conjugate beta-lyase. This PLP-dependent enzyme is proposed to catalyze the final step in the metabolic activation of cisplatin. Identification of the structure and toxicity of these conjugates further elucidates the metabolism of cisplatin to a nephrotoxin.

Possible role of membrane gamma-glutamyltransferase activity in the facilitation of transferrin-dependent and -independent iron uptake by cancer cells

BACKGROUND: The molecular mechanisms by which iron is physiologically transported trough the cellular membranes are still only partially understood. Several studies indicate that a reduction step of ferric iron to ferrous is necessary, both in the case of transferrin-mediated and transferrin-independent iron uptake. Recent studies from our laboratory described gamma-glutamyltransferase activity (GGT) as a factor capable to effect iron reduction in the cell microenvironment. GGT is located on the outer aspect of plasma membrane of most cell types, and is often expressed at high levels in malignant tumors and their metastases. The present study was aimed at verifying the possibility that GGT-mediated iron reduction may participate in the process of cellular iron uptake. RESULTS: Four distinct human tumor cell lines, exhibiting different levels of GGT activity, were studied. The uptake of transferrin-bound iron was investigated by using 55Fe-loaded transferrin, as well as by monitoring fluorimetrically the intracellular iron levels in calcein-preloaded cells. Transferrin-independent iron uptake was investigated using 55Fe complexed by nitrilotriacetic acid (55Fe-NTA complex).The stimulation of GGT activity, by administration to cells of the substrates glutathione and glycyl-glycine, was generally reflected in a facilitation of transferrin-bound iron uptake. The extent of such facilitation was correlated with the intrinsic levels of the enzyme present in each cell line. Accordingly, inhibition of GGT activity by means of two independent inhibitors, acivicin and serine/boric acid complex, resulted in a decreased uptake of transferrin-bound iron. With Fe-NTA complex, the inhibitory effect - but not the stimulatory one - was also observed. CONCLUSION: It is concluded that membrane GGT can represent a facilitating factor in iron uptake by GGT-expressing cancer cells, thus providing them with a selective growth advantage over clones that do not possess the enzyme.

Effect of gamma-glutamyl transpeptidase inhibitors on the transport of glutamate into neuronal and glial primary cultures

Inhibitors of gamma-glutamyl transpeptidase (mixture of serine and borate--13 mM, kainic acid--5 mM and 6-diazo-5-oxo-L-norleucine--2 mM) significantly suppressed glutamate uptake into cultured neurones and glial cells. The simultaneous application of any of these inhibitors with ouabain resulted in a further decline in glutamate uptake. It can be speculated that gamma-glutamyl transpeptidase significantly contributes to glutamate transport into nerve cells in the early period of brain development until the Na(+)-K(+)-gradient is fully constituted.

Targeted gene disruption reveals the role of cysteinyl leukotriene 1 receptor in the enhanced vascular permeability of mice undergoing acute inflammatory responses

The cysteinyl leukotrienes (cysLTs), leukotriene (LT) C(4), LTD(4), and LTE(4), are proinflammatory lipid mediators generated in the mouse by hematopoietic cells such as macrophages and mast cells. There are two mouse receptors for the cysLTs, CysLT(1) receptor (CysLT(1)R) and CysLT(2)R, which are 38% homologous and are located on mouse chromosomes X and 14, respectively. To clarify the different roles of the CysLT(1)R and CysLT(2)R in inflammatory responses in vivo, we generated CysLT(1)R-deficient mice by targeted gene disruption. These mice developed normally and were fertile. In an intracellular calcium mobilization assay with fura-2 acetoxymethyl ester, peritoneal macrophages from wild-type littermates, which express both CysLT(1)R and CysLT(2)R, responded substantially to 1 x 10(-6) m LTD(4) and slightly to 1 x 10(-6) m LTC(4), whereas the macrophages from CysLT(1)R-deficient mice did not respond to either LTD(4) or LTC(4). Plasma protein extravasation, but not neutrophil infiltration, was significantly reduced in CysLT(1)R-deficient mice subjected to zymosan A-induced peritoneal inflammation. Plasma protein extravasation was also significantly diminished in CysLT(1)R-deficient mice undergoing IgE-mediated passive cutaneous anaphylaxis as compared with the wild-type mice. Thus, the cysLTs generated in vivo by either monocytes/macrophages or mast cells utilize CysLT(1)R for the response of the microvasculature in acute inflammation.

Gamma-glutamyl transpeptidase activity mediates NF-kappaB activation through lipid peroxidation in human leukemia U937 cells

Gamma-glutamyl transpeptidase (GGT) is a key enzyme in the catabolism of glutathione (GSH). Recently, it has been reported that the extracellular cleavage of GSH by GGT induced the production of reactive oxygen species (ROS), suggesting that GGT plays a pro-oxidant role. In the present study, we investigated the nature of the oxidative stress generate by glutathione and GGT and the possibility that this stress affects the activity of NF-kappaB a prototypical oxidant-stress-responsive transcription factor. We found that, in the presence of iron, a natural substrate of GGT, glutathione induces lipid peroxidation in U937 cells. This induction depends on GGT activity as it is prevented by the Serine/Borate complex, a GGT inhibitor. We found that y-glutamyl transpeptidase activity induces NF-kappaB DNA binding activity, an effect which is significantly reduced by the addition of GGT inhibitors (Serine/Borate complex and Acivicin). Moreover, we show that lipid peroxidation is involved in GGT-dependent NF-kappaB activation since vitamin E, which completely inhibits GGT-induced generation of lipid peroxides, prevents the GGT-dependent NF-kappaB activation. Finally, inhibition of GGT by either the Serine/Borate complex or by Acivicin resulted in cell apoptosis. This finding suggests that GGT-mediated NF-kappaB activation plays a role in the control of apoptosis in U937 cells.

Development and evaluation of a boronate inhibitor of gamma-glutamyl transpeptidase

Gamma-glutamyl transpeptidase (gamma-GT) plays a central role in the metabolism of glutathione and is also a marker for neoplasia and cell transformation. We have investigated the compound L-2-amino-4-boronobutanoic acid (ABBA) as a structural analog of the putative ternary complex formed by the enzyme, L-serine, and borate, proposed to function as a transition state analog inhibitor. ABBA was found to be a potent inhibitor of the enzyme, with Ki = 17 nM using typical assay conditions (pH 8, gamma-glutamyl-p-nitroanilide substrate, 20 mM glycyl-glycine acceptor). ABBA is a stable amino acid analog with pK values determined from 13C and 11B NMR to be 2.3, 11.0 (amino titration), and 7.9 (boronate titration). The structural similarity to glutamate suggested that it might function as a glutamate analog for some glutamate-dependent enzymes or receptors. Transamination of pyruvate by ABBA to yield alanine in the presence of glutamic pyruvic transaminase was demonstrated by 13C NMR. The 2-keto-4-boronobutanoic acid transamination product is apparently fairly labile to hydrolysis, leading to formation of 2-ketobutanoic acid plus borate. The latter is also subsequently transaminated to yield 2-aminobutanoic acid. Both of these metabolites were observed in the 13C NMR spectrum. However, the corresponding transamination of oxaloacetate by ABBA in the presence of glutamic oxaloacetic transaminase was not observed. Effects of ABBA on the growth of cultured rat liver cell lines ARL-15C1 (nontumorigenic, low gamma-GT activity) and ARL-16T2 (tumorigenic, high gamma-GT activity) were also investigated, both in standard Williams Media as well as in a low cysteine growth medium. A high concentration (1 mM) of ABBA inhibited the growth of both cell lines in both media, with the degree of inhibition greater in the low cysteine medium. Alternatively, growth inhibition by 10 microM ABBA could be observed only in the low cysteine media. In general, there were no significant differences between the two cell lines in terms of sensitivity to ABBA.

Kinetic assay methods

The purpose of this appendix is to provide a brief review of issues important in the design of initial-rate assay methods.

Kinetic assay methods

The purpose of this unit is to provide a brief review of issues important in the design of initial-rate assay methods. General aspects of kinetic assay design are discussed, including enzyme and substrate purity, concentration and stability. Also covered are issues such as continuous versus stop-time assay formats, coupled enzyme assays, binding studies, and presentation of initial-rate data.

Identification in mice of two isoforms of the cysteinyl leukotriene 1 receptor that result from alternative splicing

Two classes of human G protein-coupled receptors, cysteinyl leukotriene 1 (CysLT(1)) and CysLT(2) receptors, recently have been characterized and cloned. Because the CysLT(1) receptor blockers are effective in treating human bronchial asthma and the mouse is often used to model human diseases, we isolated the mouse CysLT(1) receptor from a mouse lung cDNA library and found two isoforms. A short isoform cDNA containing two exons encodes a polypeptide of 339 aa with 87.3% amino acid identity to the human CysLT(1) receptor. A long isoform has two additional exons and an in-frame upstream start codon resulting in a 13-aa extension at the N terminus. Northern blot analysis revealed that the mouse CysLT(1) receptor mRNA is expressed in lung and skin; and reverse transcription-PCR showed wide expression of the long isoform with the strongest presence in lung and skin. The gene for the mouse CysLT(1) receptor was mapped to band XD. Leukotriene (LT) D(4) induced intracellular calcium mobilization in Chinese hamster ovary cells stably expressing either isoform of the mouse CysLT(1) receptor cDNA. This agonist effect of LTD(4) was fully inhibited by the CysLT(1) receptor antagonist, MK-571. Microsomal membranes from each transformant showed a single class of binding sites for [(3)H]LTD(4); and the binding was blocked by unlabeled LTs, with the rank order of affinities being LTD(4) >> LTE(4) = LTC(4) >> LTB(4). Thus, the dominant mouse isoform with the N-terminal amino acid extension encoded by an additional exon has the same ligand response profile as the spliced form and the human receptor.

The molecular characterization and tissue distribution of the human cysteinyl leukotriene CysLT(2) receptor

Cysteinyl leukotrienes (CysLTs), slow-reacting substances of anaphylaxis, are lipid mediators known to possess potent proinflammatory action. Pharmacological studies using CysLTs indicate that at least two classes of G protein-coupled receptors (GPCRs), named CysLT(1) and CysLT(2), exist; the former is sensitive and the latter is resistant to the CysLT(1) antagonists currently used to treat asthma. Although the CysLT(1) receptor gene has been recently cloned, the molecular identity of the CysLT(2) receptor has remained elusive. Here we show that the pharmacological profile of an orphan GPCR (PSEC0146) is consistent with that of the CysLT(2) receptor. In human embryonic kidney 293 cells that express the PSEC0146 cDNA, leukotriene C(4) (LTC(4)) and leukotriene D(4) (LTD(4)) induce equal increases in intracellular calcium mobilization; these increases are not affected by CysLT(1) antagonists. Additionally, [(3)H]LTC(4) specifically binds to membranes from COS-1 cells transiently transfected with PSEC0146. Large amounts of the PSEC0146 mRNA are found in human heart, placenta, spleen, and peripheral blood leukocytes but not in the lung and the trachea. Pharmacological feature and expression studies will eventually lead to a better understanding of the classification of CysLT receptors, possibly leading to a reconsideration of the pathological and physiological role of CysLTs.

Antioxidant biomarkers in freshwater bivalves, Unio tumidus, in response to different contamination profiles of aquatic sediments

Antioxidant systems were studied in the freshwater bivalve Unio tumidus transplanted from a control site to four different contaminated areas, in order to study the biological response according to the contamination characteristics. Reduced and oxidized glutathione (GSH, GSSG), the activities of antioxidant enzymes such as selenium-dependent and non-selenium-dependent glutathione peroxidases (SeGPx and non-SeGPx), and glutathione reductase (GRd) were measured in the gills and the digestive gland of the mussels after 15 days of exposure at different sites. Lipid peroxidation (LPO) was evaluated by means of malondialdehyde measurements (MDA). The four sites investigated were located in the valleys of Fensch (F), Moselle (M), Lot et Garonne (LG), and Sarthe (S). At each site, the bivalves were placed upstream (Up) from an identified pollution source (a cokery, a laundry, or a foundry) and downstream (Do), close to the effluent outfall (Do(1)) or farther (Do(2)). The goal was to study the antioxidant response in relation to the pollution gradient. Metals and congeners of PAHs, PCBs, and organochlorinated pesticides were analyzed in the river sediments of each station. The exposure of the bivalves at the most highly polluted sites or close to the pollution source led to a sharp depletion in some antioxidant parameters, namely GRd, SeGPx, and GSH. The decrease in enzyme activities could reach 80% for GRd and 70% for SeGPx, while GSH depletion could yield 70%, leading then in an induction of lipid peroxidation, either in the digestive gland or in the gills. The higher the MDA concentrations, the lower the activity of these three antioxidant parameters, suggesting that they could be biomarkers for toxicity. Yet, a depletion in these parameters was sometimes insufficient for cytotoxicity to be induced, since lipid peroxidation failed to appear in some cases where antioxidant depletion was clear, although not so severe. The response of the gills and the digestive gland was not always paralleled, which can be explained by differences in the bioavailability of pollutants. In some cases, a relationship was not found between the antioxidant response and the degree and the type of contamination in sediments, suggesting that the effects could result from nonidentified pollutants or/and be indicators of bio-availability.

Redox modulation of cell surface protein thiols in U937 lymphoma cells: the role of gamma-glutamyl transpeptidase-dependent H2O2 production and S-thiolation

The expression of gamma-glutamyl transpeptidase (GGT), a plasma membrane ectoenzyme involved in the metabolism of extracellular reduced glutathione (GSH), is a marker of neoplastic progression in several experimental models, and occurs in a number of human malignant neoplasms and their metastases. Because it favors the supply of precursors for the synthesis of GSH, GGT expression has been interpreted as a member in cellular antioxidant defense systems. However, thiol metabolites generated at the cell surface during GGT activity can induce prooxidant reactions, leading to production of free radical oxidant species. The present study was designed to characterize the prooxidant reactions occurring during GGT ectoactivity, and their possible effects on the thiol redox status of proteins of the cell surface. Results indicate that: (i) in U937 cells, expressing significant amounts of membrane-bound GGT, GGT-mediated metabolism of GSH is coupled with the extracellular production of hydrogen peroxide; (ii) GGT activity also results in decreased levels of protein thiols at the cell surface; (iii) GGT-dependent decrease in protein thiols is due to sulfhydryl oxidation and protein S-thiolation reactions; and (iv) GGT irreversible inhibition by acivicin is sufficient to produce an increase of protein thiols at the cell surface. Membrane receptors and transcription factors have been shown to possess critical thiols involved in the transduction of proliferative signals. Furthermore, it was suggested that S-thiolation of cellular proteins may represent a mechanism for protection of vulnerable thiols against irreversible damage by prooxidant agents. Thus, the findings reported here provide additional explanations for the envisaged role played by membrane-bound GGT activity in the proliferative attitude of malignant cells and their resistance to prooxidant drugs and radiation therapy.

Potentiating effects of pertussis toxin on leukotriene C4 induced formation of inositol phosphate and prostacyclin in human umbilical vein endothelial cells

Leukotriene C4 is an arachidonic acid metabolite and an important mediator of inflammation and anaphylaxis that is known to induce production of prostacyclin in endothelial cells. The goal of this study was to examine the signal transduction mechanisms activated by leukotriene C4 stimulation. Formation of inositol phosphates was measured to determine the activation of phospholipase C and pertussis toxin was used to explore the role of G-proteins. Additionally, we evaluated the role of protein kinase C in these events, especially whether there was an interaction between pertussis toxin mediated effects and the activity of protein kinase C. Leukotriene C4 induced a dose- and time-dependent formation of inositol phosphates and prostacyclin. The response to leukotriene C4 was greater than the response to leukotriene D4 even after treatment with L-serine borate complex, suggesting the presence of a specific leukotriene C4 receptor. Exposure to pertussis toxin potentiated, time-dependently, the leukotriene C4 induced formation of inositol phosphates and prostacyclin through a mechanism which was altered by manipulation of protein kinase C activity. The exact mechanism is not clear but our results are consistent with a postulated dual mechanism of phospholipase C control, in which leukotriene C4 induced stimulation is attenuated by a pertussis toxin sensitive G-protein.

Inhibitory effects in vitro of S-[2-carboxy-1-(1H-imidazol-4-yl)ethyl]-glutathione, a proposed metabolite of L-histidine, on gamma-glutamyltransferase activity

A transfer of the gamma-glutamyl moiety of S-[2-carboxy-1-(1H-imidazol-4-yl)ethyl]glutathione (I), an adduct of glutathione and L-histidine metabolite urocanic acid, has been investigated by using gamma-glutamyltransferase preparation from bovine kidney. When an equimolar mixture of two diastereomers of compound I in a phosphate buffer was allowed to react with glycylglycine in the presence of the transferase, two diastereomers of N-[S-[2-carboxy-1-(1H-imidazol-4-yl)ethyl]-L-cysteinyl]glycine (II) were formed in the same yield with each other and this was accompanied by a formation of gamma-glutamylglycylglycine. Kinetics of compound I with the transferase indicated high affinity between the two materials, while the maximal reaction velocity of the gamma-glutamyl transfer was low. Effects of compound I in vitro on the transfer of gamma-glutamyl moiety of gamma-glutamyl-p-nitroanilide to glycylglycine with the transferase were also studied, and the results indicated that the transfer was suppressed by compound I based on its competitive and non-competitive inhibitions. These results suggest that little variation in reactivities of two diastereomers of compound I as the substrate is given by the difference in stereomerism of their asymmetric carbon atoms and that inhibitory effects of compound I on the catalytic action of the transferase is of sufficient physiological importance to decrease the degradation of natural gamma-glutamyl compounds, such as glutathione and its analogs.

gamma-Glutamyl transpeptidase: catalytic mechanism and gene expression

The gamma-glutamyl transpeptidases are key enzymes in the so-called gamma-glutamyl cycle involving glutathione synthesis, the recovery of its constituents, and in the transport of amino acids. This membrane-bound ectoenzyme thus serves to regulate glutathione synthesis. This chapter deals with the active site chemistry of gamma-glutamyl transpeptidase, including the role of side-chain groups on the light subunit as well as several serine residues in the catalytic process. Also considered are genomic studies indicating (a) the presence of a single gene in mouse and rat; (b) the occurrence of multiple genes in humans; (c) the involvement of multiple promoters for gene expression; and (d) how these multiple promoters may play a role in the tissue-specific expression of gamma-glutamyl transpeptidases.

Determination of chiral catabolites from S-[2-carboxyl-1-(1H-imidazol-4-yl)ethyl]glutathione, a proposed metabolite of L-histidine, by capillary electrophoresis]

A new method for simultaneous determination of two diastereomers in each of S-[2-carboxy-1-(1H-imidazol-4-yl)ethyl]-L-cysteine (I) and N-¿S-[2-carboxy-1-(1H-imidazol-4-yl)ethyl]-L-cysteinyl¿glycine (II) was developed by electrophoresis using a neutral coated capillary with a separation buffer, pH 6.00, containing 80 mM hydroxypropyl-beta-cyclodextrin at a field strength of 500 V cm-1 at 20 degrees C. This method was applied to establishment of a catabolic pathway from S-[2-carboxy-1-(1H-imidazol-4-yl)ethyl]glutathione (III) to compound I. Incubation of either of compound II diastereomers as an enzyme substrate with rat kidney homogenate in a phosphate buffer, pH 7.4, resulted in a formation of compound I only having correspondent configurations on asymmetric carbon atoms of its molecule with those of the substrate, i.e., no occurrence of isomerization in the catabolism. Additionally, little difference in action as the substrate between two diastereomers of compound II was found. When an equimolar mixture of two diastereomers of compound III was allowed to react with the homogenate in the presence of glycylglycine, two diastereomers of compound II were formed in the same yield with each other and then these were catabolized gradually to both isomers of compound I. These results suggest that compound II is a metabolic intermediate for the formation of compound I from compound III, and that little variation in reactivities of two diastereomers of compound III as well as compound II with enzymes is given by the difference in stereoisomerism of asymmetric carbon atoms on their molecules.

Kinetic mechanism of glutathione conjugation to leukotriene A4 by leukotriene C4 synthase

The kinetic mechanism for human leukotriene (LT) C4 synthase, a membrane-bound glutathione S-transferase, which catalyzes the conjugation of glutathione (GSH) to 5,6-oxido-7,9,11, 14-eicosatetraenoic acid (LTA4), to form 5(S)-hydroxy-6(R)-S-glutathionyl-7,9,trans-11, 14-cis-eicosatetraenoic acid (LTC4) was investigated by initial rate kinetic studies in which concentrations of both substrates and the reversible dead-end inhibitor, 2-[2-[1-(4-chlorobenzyl)-4-methyl-6-[(5-phenylpyridin-2-yl)- methoxy]- 4,5-dihydro-1H-thiopyrano[2,3,4-c,d]indol-2-yl]ethoxy]butanoic acid (L-699,333) were varied. Analysis of the initial velocities of LTC4 formation in the absence of the inhibitor using non-linear regression fits of various models to the data favoured a random, rapid equilibrium mechanism, with strong substrate inhibition by LTA4, over both a compulsory ordered mechanism and a ping-pong mechanism. The estimated parameters were calculated to be Vmax = 14 +/- 4 microM/min, KLTA4 = 40 +/- 18 microM, KGSH = 0.4 +/- 0.2 mM, and a KiLTA4 = 2.3 +/- 1.7 microM for the rapid equilibrium random model. Inhibition of enzymatic activity by L-699,333 was found to be reversible as assessed by the ability of the enzyme to restore its activity by 95% upon dilution. L-699,333 was found to be a competitive inhibitor against GSH and non-competitive against LTA4. Non-linear least squares regression analysis yielded estimated parameters of Km = 0.7 +/- 0.1 mM, Vmax = 2.5 +/- 0.1 microM/min, and Ki = 0.7 +/- 0.1 microM for GSH at a fixed LTA4 concentration of 20 microM, and Km = 45 +/- 3 microM, Vmax = 4.9 +/- 0.2 microM/min, and a Ki = 5.8+/-0.4 microM for LTA4 at a fixed GSH concentration of 2 mM. The rate equation for the random equilibrium mechanism accommodates the inhibition patterns observed for L-699,333 against both substrates as revealed by kinetic fits of the inhibition data to the overall rate equation.

Regulation of enzymatic activity: one possible role of dietary boron in higher animals and humans

It is well established that vascular plants, diatoms, and some species of marine algal flagellates have acquired an absolute requirement for boron (B), although the primary role remains unknown. Discovery of naturally occurring organoboron compounds, all ionophoric macrodiolide antibiotics with a single B atom critical for activity, established at least one biochemical role of B. The unusual nature of B chemistry suggests the possibility of a variety of biological roles for B. At physiological concentrations and pH, B may react with one N group or one to four hydroxyl groups on specific biological ligands with suitable configuration and charge to form dissociable organoboron compounds or complexes. Suitable ligands include pyridine (e.g., NAD+ or NADP) or flavin (e.g., FAD) nucleotides and serine proteases (SP). B reacts with the cis adjacent hydroxyls on the ribosyl moiety of the nucleotides or, in the serine proteases, the N on the imidazole group of histidine or the hydroxyl group on the serine moiety. Reversible inhibition by B of activity of SP or oxidoreductases that require pyridine or flavin nucleotides is well known. Therefore, a proposed essential role for B is as a regulator of relevant pathways, including respiratory burst, that utilize these enzymes.

The cell-specific anti-proliferative effect of reduced glutathione is mediated by gamma-glutamyl transpeptidase-dependent extracellular pro-oxidant reactions

We have shown earlier that extracellular GSH can exert a cell-specific growth-inhibitory effect on human tumor cells. In the present study, 2 human ovarian carcinoma cell lines (A2780 and IGROV-1) were used to investigate the biochemical basis of the GSH growth-inhibitory effect. Whereas cells were resistant, A2780 cells were sensitive to a 1 hr exposure to GSH, as assessed by the growth inhibition assay. Analysis of relevant GSH-dependent enzymes indicated that A2780 cells had low level of GSH S-transferase, glutathione reductase and gamma-glutamyl transpeptidase (gamma-GT) activities in comparison with those of IGROV-1 cells, and GSH peroxidase activity was undetectable in A2780 cells. The GSH effect was reversed by catalase and by dithiothreitol, indicating the occurrence of oxidative phenomena resulting in the impairment of critical cellular thiols. Indeed treatment of cells with H(2)O(2) also resulted in growth inhibition, which was more marked in A2780 cells. The gamma-glutamyl acceptor glycylglycine, a co-substrate for gamma-GT, potentiated the growth-inhibitory effect of GSH, which in contrast was decreased by the gamma-GT inhibitors, serine-borate complex and acivicin, suggesting that the production of reactive forms of oxygen (probably H(2)O(2)) was mediated by cysteinyl-glycine after GSH hydrolysis. The results support that the growth-inhibitory effect of low GSH concentration is the result of oxidative damage related to extracellular GSH metabolism.

Synthesis and activity of the glutathione analogue gamma-(L-gamma-azaglutamyl)-L-cysteinyl-glycine

The backbone-modified glutathione analogue gamma-(L-gamma-azaglutamyl)-L-cysteinyl-glycine 7, characterized by the presence of a NHCONH urea linkage deriving from the replacement of the native Glu gamma-CH2 with the aza (NH) group, was synthesized and fully characterized by FAB-MS, 1H- and 13C-NMR. Potential of 7 and its oxidized form 6 as gamma-glutamyltransferase inhibitors was investigated. Both compounds 7 and 6 were found to be competitive inhibitors of hog kidney gamma-glutamyltransferase (EC 2.3.2.2.) by binding at the donor site: the reduced analogue is a more efficient inhibitor than glutathione of the gamma-glutamyl transfer reaction. Inhibition at the acceptor site, which is also present, appears to be more complex. In particular, un-competitive inhibition is observed for compound 7. The results indicate that gamma-azapeptides of type 7 may represent interesting targets in the search for stable inhibitors of gamma-glutamyltransferases.

Involvement of Ser-451 and Ser-452 in the catalysis of human gamma-glutamyl transpeptidase

The serine residue required for catalysis of gamma-glutamyl transpeptidase was identified by site-specific mutagenesis of the conserved serine residues on the basis of sequence alignment of the light subunit of human, rat, pig and two bacterial enzymes. Recombinant human gamma-glutamyl transpeptidases with replacements of these serine residues by Ala were expressed using a baculovirus-insect cell system. Substitutions of Ala at Ser-385, -413 or -425 yielded almost fully active enzymes. However, substitutions of Ala at Ser-451 or -452 yielded enzymes that were only about 1% as active as the wild-type enzyme. Further, their double mutant is only 0.002% as active as the wild type. Kinetic analysis of transpeptidation using glycylglycine as acceptor indicates that the Vmax values of Ser-451 and -452 mutants are substantially decreased (to about 3% of the wild type); however, their Km values for L-gamma-glutamyl-p-nitroanilide as donor were only increased about 5 fold compared to that of the wild type. The double mutation of Ser-451 and -452 further decreased the Vmax value to only about 0.005% of the wild type, while this mutation produced only a minor effect (2-fold increase) on the Km value for the donor. The kinetic values for the hydrolysis reaction of L-gamma-glutamyl-p-nitroanilide in the mutants followed similar trends to those for transpeptidation. The rates of inactivation of Ser-451, -452 and their double mutant enzymes by acivicin, a potent inhibitor, were less than 1% that of the wild-type enzyme. The Ki value of the double mutant for L-serine as a competitive inhibitor of the gamma-glutamyl group is only 9 fold increased over that of the wild type, whereas the Ki for the serine-borate complex, which acts as an inhibitory transition-state analog, was more than 1,000 times higher than for the wild-type enzyme. These results suggest that both Ser-451 and -452 are located at the position able to interact with the gamma-glutamyl group and participate in catalysis, probably as nucleophiles or through stabilization of the transition state.

Different sites of acivicin binding and inactivation of gamma-glutamyl transpeptidases

Acivicin is a potent inhibitor of gamma-glutamyl transpeptidase (EC 2.3.2.2), an enzyme of importance in glutathione metabolism. Acivicin inhibition and binding are prevented by gamma-glutamyl substrates and analogs (e.g., serine plus borate), consistent with the previous postulate that acivicin and substrates bind to the same enzyme site. Inactivation of rat kidney transpeptidase by acivicin leads to its binding as an ester to Thr-523. The pig enzyme, which has Ala-523 in place of Thr-523, is inhibited by acivicin with esterification at Ser-405. The human enzyme has Thr-524 (corresponding to Thr-523 in rat); its inactivation leads to esterification of Ser-406 (corresponding to Ser-405 in rat and pig). Hydroxylamine treatment of the acivicin-inactivated enzymes restores activity and releases the acivicin-derived threo-beta-hydroxyglutamate moiety. The findings indicate that there are significant structural differences between the active site region of the rat enzyme and the active site regions of the human and pig. Human mutant enzymes in which Thr-524 and Ser-406 were replaced by Ala, separately and together, are enzymatically active, indicating that these amino acid residues are not required for catalysis. However, esterification of these residues (and of another near the active site) effectively blocks the active site or hinders its function. Acivicin can bind at enzyme sites that are close to that at which gamma-glutamylation occurs; it may bind at the latter site and then be transesterified to another enzyme site.

Modulation of quinol/quinone-thioether toxicity by intramolecular detoxication

A variety of cytotoxic, mutagenic, and carcinogenic conjugates of GSH require processing by enzymes of the mercapturic acid pathway to produce toxicity. However, metabolism of quinone-thioethers by gamma-GT can result in either activation or detoxication. For example, inhibition of gamma-GT completely protects against the nephrotoxicity caused by 2-bromo-bis-(glutathion-S-yl)hydroquinone and 2,3,5-tris-(glutathion-S-ly)hydroquinone, whereas the same protocol potentiates the nephrotoxicity of 2,5-dichloro-3-(glutathion-S-yl)hydroquinone and 2,5,6-trichloro-3-(glutathion-S-yl)hydroquinone. Which of these two scenarios occur as a consequence of metabolism by gamma-GT appears to be determined by the relative rate at which the product is transported into cells and/or interacts with cellular constituents, and the rate which the product undergoes intramolecular detoxication (cyclization) to a 1,4-benzothiazine. The same reaction may also explain why the mercapturic acid metabolite of menadione is nephrotoxic following systemic administration, whereas the GSH conjugate is without activity. Species differences exist in susceptibility to both 2-bromo-bis-(glutathion-S-ly)hydroquinone and 2,3,5-tris(glutathion-S-ly)hydroquinone induced nephrotoxicity. In this case, however, susceptibility does not correlate with renal gamma-GT activity, but rather to differences in the rate at which the corresponding cysteine and N-acetylcysteine conjugates undergo N-acetylation/N-deacetylation cycling. Thus the guinea pig--which is the only other rodent species (in addition to the rat), that is susceptible to 2-bromo-bis-(glutathion-S-ly)hydroquinone and 2,3,5-tris-(glutathion-S-ly)hydroquinone mediated nephrotoxicity--expresses the lowest activity of renal gamma-GT but exhibits the highest N-deacetylation:N-acetylation ratio. Differences in kinetics of these two reactions therefore contribute to species susceptibility. The toxicity of quinol/quinone thioethers is dependent upon a number of physiological, biochemical, and electrochemical factors. The rates at which quinol-thioethers undergo oxidation, with the concomitant generation of reactive oxygen species (IV, Fig. 1), macromolecular arylation (V, Fig. 1), intramolecular cyclization (VI, Fig. 1), and acetylation-deacetylation cycling (III, Fig. 1) is dependent upon the substrate in question. All these factors will contribute to the cell, tissue, and species susceptibility of this interesting class of GSH conjugates.

gamma-Glutamyltranspeptidase-catalysed acyl-transfer to the added acceptor does not proceed via the ping-pong mechanism

Acyl-transfer catalysed by gamma-glutamyltranspeptidase from bovine kidney was studied using gamma-L- and gamma-D-Glu-p-nitroanilide as the donor and GlyGly as the acceptor. The transfer of the gamma-Glu group to GlyGly was shown to be accompanied by transfer of the gamma-Glu group to water (hydrolysis). The results were compared with acyl-transfer catalysed by the representative serine protease, alpha-chymotrypsin. The main difference between the kinetic mechanism of the acyl-transfer reactions catalysed by these enzymes, which contain an active-site serine and form an acyl-enzyme intermediate but belong to different enzyme classes, was found to consist in the role of the enzyme-donor-acceptor complex. This complex is not formed at any acceptor concentrations in the acyl-transfer reactions catalysed by the serine proteases. In contrast, in the gamma-glutamyltranspeptidase-catalysed acyl-transfer the pathway going through the ternary enzyme-donor-acceptor complex formed from the enzyme-acceptor complex becomes the main pathway of the transfer reaction even at moderate acceptor concentrations. As a result, gamma-glutamyltranspeptidase catalysis follows a sequential mechanism with random equilibrium addition of the substrates and ordered release of the products. The second distinction concerns the inhibitory effect of the acceptor. In the case of alpha-chymotrypsin this was the result of true inhibition, i.e. a dead-end formation of the enzyme-acceptor complex. A salt effect caused by the acceptor was the rationale of a similar effect observed in acyl-transfer catalysed by gamma-glutamyltranspeptidase.

Abnormalities of retinal metabolism in diabetes or galactosemia. II. Comparison of gamma-glutamyl transpeptidase in retina and cerebral cortex, and effects of antioxidant therapy

Levels of the intracellular antioxidant, glutathione, become subnormal in retina in diabetes or experimental galactosemia. In order to investigate the cause and significance of this abnormality, activity of gamma-glutamyl transpeptidase (an enzyme important in the synthesis and degradation of glutathione) and levels of reduced glutathione have been measured in retinas of diabetic rats and dogs and of experimentally galactosemic rats and dogs. Retinal gamma-glutamyl transpeptidase activity and glutathione level were significantly less than normal after 2 months of diabetes or galactosemia. In contrast, cerebral cortex from the same diabetic rats and galactosemic rats showed no significant reduction in either gamma-glutamyl transpeptidase activity or glutathione level. These different responses of the two tissues to hyperglycemia might help account for the difference in microvascular disease in these two tissues in diabetes. Consumption of the antioxidants, ascorbic acid (1.0%) plus alpha-tocopherol (0.1%), by diabetic rats and galactosemic rats inhibited the decrease of gamma-glutamyl transpeptidase activity and glutathione levels in retina, suggesting that defects in glutathione regulation in the retina are secondary to hyperglycemia-induced 'oxidative stress'.

Gamma-glutamyltranspeptidase expression regulates the growth-inhibitory activity of the anti-tumor prodrug gamma-L-glutaminyl-4-hydroxy-3-iodobenzene

gamma-L-glutaminyl-4-hydroxy-3-iodobenzene (I-GHB), a novel iodinated analog of gamma-L-glutaminyl-4-hydroxybenzene (GHB), demonstrates greater anti-tumor activity in human and in murine melanoma cell lines. These phenolic amides are substrates for gamma-glutamyltranspeptidase (GGTP; E.C. 2.3.2.2), a cell-membrane-associated ecto-enzyme which is elevated in a number of tumor systems. We now present data to show that the growth-inhibitory activity of I-GHB and GHB may be mediated via GGTP-catalyzed reactions. The growth-inhibitory activity of I-GHB and GHB in pigmented B16-BL6 melanoma cells was blocked significantly by rabbit anti-rat GGTP polyclonal antibodies. The combination of L-serine and sodium borate, a specific transition-state inhibitor of GGTP, as well as acivicin, a glutamine antagonist and irreversible GGTP inhibitor, inhibited the killing of BL6 cells by GHB and I-GHB. To further define the role of GGTP expression in the regulation of phenolic amide cytotoxicity, GGTP-negative Chinese hamster ovary cells (CHO-K1) were transfected with a functional rat renal cDNA representing the full-length GGTP transcript. I-GHB and GHB were significantly more cytotoxic in GGTP cDNA transfected Chinese hamster ovary (CHO-K1-GGTP) cells than in non-transfected CHO-K1 cells. The combination of L-serine and sodium borate blocked the cytotoxic activity of these pro-drugs and also inhibited GGTP-catalyzed formation of polymerized products from these phenolic amides in intact BL6 melanoma and CHO-K1-GGTP cells. Furthermore, melanin formation from GHB was not observed in non-transfected CHO-K1 cells lacking GGTP expression. The combined data strongly suggest that GGTP-catalyzed hydrolysis of the anti-tumor pro-drugs I-GHB and GHB to 4-aminophenols mediates the expression of antitumor activity.